Emperor Gaozong of Tang (Chinese: 唐高宗; pinyin: Táng Gāozōng) (21 July 628[3] – 27 December 683[4]), personal name Li Zhi (李治), was the third emperor of the Tang Dynasty in China, ruling from 649 to 683 (although after January 665 much of the governance was in the hands of his second wife Empress Wu (later known as Wu Zetian)). Emperor Gaozong was the son of Emperor Taizong and Empress Zhangsun.

Emperor Gaozong was aided in his rule by Empress Wu during the later years of his reign after a series of strokes left him incapacitated. Emperor Gaozong delegated all matters of state to his wife and after he died in 683, power fell completely into the hands of Empress Wu, who subsequently became the only Empress regnant in Chinese history. After his death, he was interred at the Qianling Mausoleum along with Wu Zetian.

Historians have generally viewed Emperor Gaozong as a weak ruler, inattentive to the business of the state and leaving such business to his powerful wife Empress Wu. During the first part of his reign, Tang territorial gains, which started with his father Emperor Taizong, continued, including the conquest of Baekje, Goguryeo, and Western Tujue, but throughout the 670s, much of those gains were lost to Tufan, Silla, Khitan, and Balhae. Further, territory previously conquered that belonged to both Eastern and Western Tujue were subjected to repeated rebellions.

Li Zhi was born in 628. He was the ninth son of his father, Emperor Taizong, and the third son of his mother, Emperor Taizong's wife Empress Zhangsun. In 631, he was created the Prince of Jin. In 633, he was made commandant of Bing Prefecture (并州, roughly modern Taiyuan, Shanxi), but remained at the capital Chang'an rather than reporting to Bing Prefecture. When Empress Zhangsun died in 636, Emperor Taizong was particularly touched by the grief that Li Zhi displayed, and from that point on particularly favored him. Sometime while he was the Prince of Jin, at the recommendation of his grand aunt Princess Tong'an, he married the grandniece of Princess Tong'an's husband Wang Yu (王裕) as his wife and princess.

Meanwhile, Li Zhi's two older brothers by Empress Zhangsun, Li Chengqian the Crown Prince and Li Tai the Prince of Wei, were locked in an intense rivalry, as Li Tai was favored by Emperor Taizong for his talent and was trying to displace Li Chengqian. Li Chengqian, in fear, entered into a conspiracy with the general Hou Junji, his uncle Li Yuanchang (李元昌) the Prince of Han, the imperial guard commander Li Anyan (李安儼), and his brothers-in-law Zhao Jie (趙節, who was also his cousin) and Du He (杜荷) to overthrow Emperor Taizong. The plot was discovered in 643, and Emperor Taizong deposed Li Chengqian. He was initially going to make Li Tai the new crown prince, but later began to believe that Li Tai's machinations were responsible for Li Chengqian's downfall. The powerful chancellor Zhangsun Wuji – Empress Zhangsun's brother—suggested that he make Li Zhi crown prince, a possibility that Li Tai was apprehensive about. Li Tai tried to intimidate Li Zhi, who had been friendly with Li Yuanchang, by pointing out to Li Zhi that Li Yuanchang had been part of the plot and that he should be concerned for himself. When Emperor Taizong noticed Li Zhi worrying about this and was told by Li Zhi of Li Tai's intimidation, Emperor Taizong's mind became set. He exiled Li Tai, and on 30 April, 643,[5] he created Li Zhi the new crown prince. He made Zhangsun and two other senior chancellors, Fang Xuanling and Xiao Yu, senior advisors to Li Zhi, and made another chancellor, Li Shiji, the head of Li Zhi's household. At the advice of another key official, Liu Ji, who pointed out that the crown prince needed to have a group of well-learned scholars that he was close to, Emperor Taizong appointed Liu, as well as Cen Wenben, Chu Suiliang, and Ma Zhou, to serve as Li Zhi's friends and advisors.

Late in 643, Emperor Taizong issued an edict to select beautiful women among good households to serve as Li Zhi's concubines. However, after Li Zhi declined such treatment, Emperor Taizong cancelled the edict. However, during his years as crown prince, he was said to have favored his concubine Consort Xiao, having two daughters (the later Princesses Yiyang and Gao'an) and one son (Li Sujie) with her, much to the chagrin of his wife Crown Princess Wang, who was childless and jealous for Consort Xiao. Three other concubines of his bore his other sons Li Zhong, Li Xiao (李孝), and Li Shangjin (李上金).

Around the same time, however, Emperor Taizong also became concerned that Li Zhi, who was considered kind but weak in character, would not be strong enough to be an emperor, and secretly discussed with Zhangsun Wuji the possibility of making another son by his concubine Consort Yang (daughter of Emperor Yang of Sui), Li Ke the Prince of Wu, crown prince. Zhangsun repeatedly opposed the idea, and Emperor Taizong did not carry this out.

In 645, when Emperor Taizong launched a campaign against Goguryeo, he took Li Zhi with him to Ding Prefecture (定州, roughly modern Baoding, Hebei) and then left Li Zhi there to be in charge of logistics, before heading to the front himself. He also left senior officials Gao Shilian, Liu Ji, Ma Zhou, Zhang Xingcheng, and Gao Jifu to assist Li Zhi. After the campaign ended in failure later that year, as Emperor Taizong was leading the army back from the front, Li Zhi went to meet him at Linyu Pass (臨渝關, now Shanhai Pass). Emperor Taizong suffered an injury during the campaign, and Li Zhi was said to have, as Emperor Taizong's conditions were getting worse, sucked the pus out of his wound, until Emperor Taizong recovered somewhat. In 646, with Emperor Taizong still recovering, he transferred some of the imperial authorities to Li Zhi. Li Zhi stayed at the imperial palace and attended to Emperor Taizong in his illness. That year, when Emperor Taizong was due to visit Ling Prefecture (靈州, roughly modern Yinchuan, Ningxia) to meet with a number of tribal chiefs who were formerly vassals of Xueyantuo—which had collapsed under Tang and Huige attacks earlier that year—he was set to take Li Zhi with him, but at Zhang's suggestion left Li Zhi in charge at Chang'an instead, to allow Li Zhi to become more familiar with the important affairs of state in his absence. After Emperor Taizong returned from Ling Prefecture, he retained for himself the authorities over imperial worship, state guests, military, the commissioning of officers of higher than the fifth rank, and executions, and transferred all other authorities to Li Zhi.

In 647, a commoner named Duan Zhichong (段志沖) submitted a petition to Emperor Taizong, asking him to pass the throne to Li Zhi. Li Zhi, concerned that Emperor Taizong might be offended, was worried and grieving, and Zhangsun suggested that Duan be executed. Emperor Taizong did not take offense and did not punish Duan or Li Zhi. Meanwhile, Li Zhi began to build a Buddhist temple named Daci'en Temple (大慈恩寺) in commemoration of his mother Empress Zhangsun, and the temple was completed in 648.

In 649, while at the summer palace Cuiwei Palace (翠微宮, in the Qin Mountains), Emperor Taizong was gravely ill, and he, while impressed with Li Shiji's abilities, was concerned that Li Shiji was too able and would not submit to Li Zhi. He stated to Li Zhi:

Li Shiji is full of ability and wisdom, but you had done him no favors, and it may be difficult for him to be faithful to you. I am going to exile him now. If he leaves immediately, promote him to be Puye [(僕射, head of the important examination bureau of the government)] and trust him after I die. If he hesitates, execute him.

He then demoted Li Shiji to the post of the commandant of Die Prefecture (疊州, roughly modern Gannan Tibetan Autonomous Prefecture, Gansu). Li Shiji, realizing what was happening, after receiving the order, departed without hesitation.

As Emperor Taizong's illness was getting more serious, Li Zhi continuously attended to him and wept constantly, often going without food, which touched Emperor Taizong greatly. Emperor Taizong entrusted Li Zhi to Zhangsun and Chu Suiliang, and then died on 10 July, 649.[6] Initially, Li Zhi was so mournful that he could not carry out any actions other than holding onto the necks of Zhangsun and Chu. Zhangsun, while mourning himself, reminded Li Zhi that he was now in charge of the empire and must act accordingly. Zhangsun also ordered that Emperor Taizong's death not be announced for the time being, and then, the next day, accompanied Li Zhi back to Chang'an. Zhangsun issued several edicts in Emperor Taizong's name—including making Yu Zhining, Zhang, and Gao Jifu chancellors. Two days later, Emperor Taizong's death was officially announced. On 15 July,[7] Li Zhi took the throne (as Emperor Gaozong).

Emperor Gaozong's first move as emperor was to cancel a second campaign against Goguryeo that Emperor Taizong had planned for later 649. While Li Tai was disallowed from attending Emperor Taizong's funeral, Emperor Gaozong permitted him to again have a staff and be allowed to use wagons, clothes, and foods of high quality. Emperor Gaozong created his wife Crown Princess Wang empress and created her father Wang Renyou (王仁祐) the Duke of Wei. It was said that early in Emperor Gaozong's reign, he greatly respected both his uncle Zhangsun Wuji and Chu Suiliang and followed their advice, and that therefore, during this part of his reign, the government was organized well and the people were comforted, much like during the reign of Emperor Taizong, although in winter 650, Chu was accused of forcibly purchasing private land and paying below-market price, and was demoted to be a prefectural prefect. (Chu eventually returned to power in 653.)

Also in 650, the general Gao Kan (高侃) – whose army had been launched by Emperor Taizong against the newly reconstituted Eastern Tujue under Chebi Khan Ashina Hubo prior to Emperor Taizong's death—captured Ashina Hubo and brought him back to Chang'an. Emperor Gaozong spared Ashina Hubo and made him a general, putting his people directly under Tang rule. Meanwhile, with two of the Xiyu states previously conquered by Tang and governed by Tang-installed kings, Qiuzi and Yanqi in disturbance, Emperor Taizong returned their previously-captured kings, Bai Helibushibi (白訶黎布失畢) and Long Tuqizhi (龍突騎支) respectively, to their thrones.

In 651, the Western Tujue prince Ashina Helu, who had sought and received protection from Emperor Taizong, broke away from Tang and defeated Western Tujue's Yipishekui Khan, taking over Western Tujue himself and no longer subordinate under Tang. In fall 651, Ashina Helu attacked Tang's Ting Prefecture (庭州, roughly modern Changji Hui Autonomous Prefecture, Xinjiang), and Emperor Gaozong responded by commissioning the generals Liang Jianfang (梁建方) and Qibi Heli (契苾何力) to attack Ashina Helu. Liang and Qibi achieved some victories against Ashina Helu's general Zhuxie Guzhu (朱邪孤注), but then withdrew without engaging Ashina Helu.

Meanwhile, as Empress Wang was sonless, her uncle, the chancellor Liu Shi, suggested to her that she ask Emperor Gaozong to create his oldest son Li Zhong, whose mother Consort Liu was of low birth and therefore she considered nonthreatening, crown prince so that Li Zhong would be grateful of her in the future. Liu also persuaded Zhangsun to suggest the idea as well, and Emperor Gaozong. in fall 652, Emperor Gaozong created Li Zhong crown prince. By this point, however, Empress Wang was facing a major threat from another romantic rival. When Emperor Gaozong was crown prince, he had been attracted by the beauty of one of Emperor Taizong's concubines, Consort Wu. After Emperor Taizong's death, all of his concubines who did not bear sons were housed at Ganye Temple (感業寺) to be Buddhist nuns. In either 650 or 651,[8] when Emperor Gaozong was visiting Ganye Temple to offer incense to Buddha, when he saw Consort Wu. Both of them wept. When Empress Wang heard this, she, wanting to divert Emperor Gaozong's favor from Consort Xiao, secretly instructed Consort Wu to grow her hair back, while suggesting to Emperor Gaozong that he take her as a concubine. Consort Wu was intelligent and full of machinations, and therefore, when she first returned to the palace, she acted humbly and flattered Empress Wang, who trusted her greatly and recommended her to Emperor Gaozong. Soon, Emperor Gaozong became enamored with Consort Wu.

Meanwhile, Emperor Gaozong's sister Princess Gaoyang and her husband Fang Yi'ai (房遺愛, Fang Xuanling's son), were implicated in 652 of conspiring with another brother-in-law Chai Lingwu (柴令武), the general Xue Wanche (薛萬徹) and Emperor Gaozong's uncle Li Yuanjing (李元景) the Prince of Jing to make Li Yuanjing emperor. Fang, knowing that Zhangsun had long been apprehensive of Li Ke, falsely implicated Li Ke in the plot as well, hoping to ingratiate Zhangsun sufficiently that he would be spared. nevertheless, in spring 653, at the suggestion of Zhangsun and Cui Dunli – despite Emperor Gaozong's initial inclination to spare Li Yuanjing and Li Ke—Emperor Gaozong ordered that Fang, Xue, and Chai be executed, and that Li Yuanjing, Li Ke, and the Princesses Gaoyang and Baling (Chai's wife) be forced to commit suicide. Zhangsun took this opportunity to accuse several other officials friendly with Fang or hostile to him—the chancellor Yuwen Jie, Li Daozong the Prince of Jiangxia, and the general Zhishi Sili (執失思力) – of being friendly with Fang and had them exiled. He also deposed and exiled Li Ke's mother Consort Yang and Consort Yang's other son Li Yin (李愔) the Prince of Shu, as well as Fang's brother Fang Yizhi (房遺直) and Xue's brother Xue Wanbei (薛萬備).

By 654, both Empress Wang and Consort Xiao had lost favor with Emperor Gaozong, and the former romantic rivals joined forces against Consort Wu, but to no avail, and as a sign of his love to Consort Wu, in 654 he conferred posthumous honors on her father Wu Shihuo (武士彠). Later that year, after Consort Wu gave birth to a daughter, Empress Wang visited her—and after Empress Wang left, Consort Wu killed her own daughter and then implicated Empress Wang in the killing. In anger, Emperor Gaozong considered deposing Empress Wang and replacing her with Consort Wu, but wanted to make sure that the chancellors would support this, and so visited Zhangsun's house with Consort Wu, awarding him with much treasure, but when he brought up the topic that Empress Wang was sonless (as an excuse for deposing her), Zhangsun repeatedly found ways to divert the conversation, and subsequent visits by Consort Wu's mother Lady Yang and the official Xu Jingzong, who was allied with Consort Wu, to seek support from Zhangsun were also to no avail.

In summer 655, Consort Wu accused Empress Wang and her mother Lady Liu of using witchcraft. In response, Emperor Gaozong barred Lady Liu from the palace and demoted Liu Shi. Meanwhile, a faction of officials began to form around Consort Wu, including Li Yifu, Xu, Cui Yixuan (崔義玄), and Yuan Gongyu (袁公瑜). On an occasion in fall 655, Emperor Gaozong summoned the chancellors Zhangsun, Li Shiji (who by now was using the name Li Ji to observe naming taboo for Emperor Taizong's name Li Shimin), Yu Zhining, and Chu to the palace—which Chu deduced to be regarding the matter of changing the empress. Li Ji claimed an illness and refused to attend. At the meeting, Chu vehemently opposed deposing Empress Wang, while Zhangsun and Yu showed their disapproval by silence. Meanwhile, other chancellors Han Yuan and Lai Ji also opposed the move, but when Emperor Gaozong asked Li Ji again, Li Ji's response was, "This is your family matter, Your Imperial Majesty. Why ask anyone else?" Emperor Gaozong therefore became resolved. He demoted Chu to be a commandant at Tan Prefecture (roughly modern Changsha, Hunan), and then deposed both Empress Wang and Consort Xiao, putting them under arrest and creating Consort Wu empress instead to replace Empress Wang. (Later that year, Empress Wang and Consort Xiao were killed on orders by the new Empress Wu after Emperor Gaozong showed signs of considering their release.) Then, at Xu's suggestion, in spring 656, Emperor Gaozong demoted Li Zhong to be the Prince of Liang and created Empress Wu's oldest son Li Hong the Prince of Dai crown prince instead.

In 655 as well, Emperor Gaozong commissioned the general Cheng Zhijie (程知節) to attack Ashina Helu, but while the campaign saw some victories over Western Tujue's substituent tribes Geluolu (歌邏祿) and Chuyue (處月), it was hindered by Cheng's inability to restrain his assistant Wang Wendu (王文度) from pillaging and inappropriately halting the army. After the campaign ended in early 657, both Cheng and Wang were deposed from their offices.

In 657, Emperor Gaozong commissioned the general Su Dingfang, who had served under Cheng Zhijie in the earlier failed campaign, to command a campaign against Ashina Helu, assisted by Ren Yaxiang and Xiao Siye (蕭嗣業). They were joined by the Western Tujue chieftains Ashina Mishe and Ashina Buzhen, who had submitted to Tang during Emperor Taizong's reign. The campaign caught Ashina Helu by surprise, and Su defeated him in several battles, causing him to flee to the kingdom Shi (石國, centering modern Tashkent, Uzbekistan), which arrested him and delivered him to Su, thus largely ending Western Tujue as an organized state. (Emperor Gaozong would try to continue Western Tujue's existence as a vassal stage by dividing it in half and creating Ashina Mishe and Ashina Buzhen as khans of the two halves.)

Meanwhile, Xu Jingzong and Li Yifu, aligned with Empress Wu, began to carry out a campaign of reprisal on her behalf. In 657, they accused Han Yuan and Lai Ji of plotting treason with Chu Suiliang, who was then serving as the commandant at Gui Prefecture (桂州, roughly modern Guilin, Guangxi). Emperor Gaozong demoted Han and Lai to be prefects of distant prefectures, and demoted Chu and Liu Shi to even more distant prefectures—in Chu's case, to the extremely distant Ai Prefecture (愛州, roughly modern Thanh Hóa Province, Vietnam), and Chu's subsequent petition, sent from Ai Prefecture, pleading with Emperor Gaozong, fell on deaf ears.

Empress Wu's reprisals did not end there. In 659, Zhangsun Wuji became the next target. At that time, two low level officials, Wei Jifang (韋季方) and Li Chao (李巢) had been accused of improper associations, and when Emperor Gaozong put Xu and Xin Maojiang of investigating, Xu falsely accused Wei and Li to be part of a treasonous plot by Zhangsun. Emperor Gaozong, without meeting with Zhangsun, believed Xu, and put Zhangsun under house arrest in exile at Qian Prefecture (黔州, roughly modern southeastern Chongqing). Xu further implicated Chu, Liu, Han, and Yu Zhining in the plot as well. Yu was removed from his post. Chu, who had died in 658, was posthumously stripped of all titles, and his sons Chu Yanfu (褚彥甫) and Chu Yanchong (褚彥沖) were executed. Orders were also issued to execute Liu and Han, although Han died before the execution order reached his location. Meanwhile, Zhangsun, once he reached his place of exile, was forced to commit suicide. It was said that after Han's and Lai's deaths, no official dared to criticize the emperor any further.

Also in 659, a vassal of Western Turkic Kaganate (Pin. Tujue), Duman (都曼), the commander of the Esegels (aka Izgil, , Ch. Asijie, Sijie 思結) Tribe, rebelled against Tang occupation, along with Western Tujue's subject kingdoms Shule (疏勒), Zhujupo (朱俱波), and Yebantuo (謁般陀) (all in modern Kashgar, Xinjiang). The joint forces commanded by Duman quickly defeated the Tang vassal Yutian (于田, in modern Hotan, Xinjiang). In winter 659, Emperor Gaozong sent Su Dingfang against Duman, and once he arrived in the vicinity of Duman's army, he selected 10,000 infantry soldiers and 3,000 cavalry soldiers and made a surprise attack on Duman. When he arrived at Duman's headquarters, Duman was surprised, and after Su initially defeated Duman, Duman was forced to withdraw within the city. Su put the city under siege, and Duman surrendered. In spring 660, Su took Duman back to the eastern capital Luoyang, where Emperor Gaozong was at the time, to present Duman to him. Some officials requested that Duman be executed, but Su made a plea on Duman's behalf—that he had promised Duman life before Duman surrendered—and Emperor Gaozong stated that while under the law, Duman should die, he would honor Su's promise, and so he spared Duman.

On the other end of the Tang atmosphere of influence, on the Korean Peninsula, Baekje had been attacking the Tang ally Silla. Once Su returned from the Esegel campaign, Emperor Gaozong commissioned him to head over the sea to attack Baekje, in conjunction with Silla. Su quickly captured the Baekje capital Sabi, forcing Baekje's King Uija and his crown prince Buyeo Yung to surrender. Emperor Gaozong ordered that Baekje be annexed as Tang territory. Emperor Gaozong then followed up by commissioning Su, along with Qibi Heli, Liu Boying (劉伯英), and Cheng Mingzhen (程名振), to attack Goguryeo.

Meanwhile, just after Su Dingfang left Baekje territory to attack Goguryeo, the Buddhist monk Dochim (道琛) and the former Baekje general Buyeo Boksin rose to try to revive Baekje. They welcomed the Baekje prince Buyeo Pung back from Japan to serve as king, with Juryu (주류, 周留, in modern Seocheon County, South Chungcheong) as their headquarters. They put the Tang general Liu Renyuan (劉仁願) under siege in Sabi. Emperor Gaozong sent the general Liu Rengui, who had previously been demoted to commoner rank for offending Li Yifu, with a relief force, and Liu Rengui and Liu Renyuan were able to fight off the Baekje resistance forces' attacks, but were themselves not strong enough to quell the rebellion, and so for some time the armies were in stalemate.

Meanwhile, Su advanced on the Goguryeo capital Pyongyang and put it under siege, but was unable to capture it quickly. In spring 662, after the general Pang Xiaotai (龐孝泰) was defeated by Goguryeo forces at Sasu River (蛇水, probably Botong River) and was killed along with his 13 sons, Su ran into harsh snowstorms and withdrew.

Around the same time, after the death of the Huige chief Yaoluoge Porun (藥羅葛婆閏), who had been obedient to Tang, Yaoluoge Porun's nephew Yaoluoge Bisudu (藥羅葛比粟毒) rose in rebellion with the Tongluo (同羅) and Pugu (僕固) tribes in conjunction with other Tiele Confederation tribes. Emperor Gaozong sent the general Zheng Rentai (鄭仁泰) to attack the Tiele, but while Zheng was initially victorious, his officers became bogged down in pillaging and eventually suffered great losses after being caught in poor weather. Emperor Gaozong instead sent Qibi, who was ethnically Tiele, assisted by Jiang Ke, to Tiele to try to persuade them to surrender. Qibi was able to do so, and rebel leaders were arrested and turned over to Tang. Qibi executed them and ended the rebellion.

Meanwhile, for reasons unknown, also in 662 Emperor Gaozong sent the general Su Haizheng (蘇海政) to attack Qiuzi and ordered Ashina Mishe and Ashina Buzhen to assist him. Ashina Buzhen, who had a rivalry with Ashina Mishe, falsely informed Su that Ashina Mishe was set to rebel and would attack the Tang army, and Su responded by ambushing Ashina Mishe, killing him and his chief assistants. The Western Tujue tribes, angry over Ashina Mishe's death, largely turned away from Tang and submitted to Tufan instead, and when Ashina Buzhen died later that year, Tang influence in the region was greatly reduced.

During these years, Li Yifu had been, due to favors from Emperor Gaozong and Empress Wu, exceedingly powerful, and he grew particularly corrupt. In 663, after reports of Li Yifu's corruption were made to Emperor Gaozong, Emperor Gaozong had Liu Xiangdao and Li Ji investigate, finding Li Yifu guilty. Li Yifu was removed from his post and exiled, and would never return to Chang'an.

During the years, Empress Wu had repeatedly, in her dreams, seen Empress Wang and Consort Xiao, in the states they were after their terrible deaths, and she came to believe that their spirits were after her. For that reason, Emperor Gaozong started remodeling a secondary palace, Daming Palace (大明宮), into Penglai Palace (蓬萊宮), and when Penglai Palace's main hall, Hanyuan Hall (含元殿), was completed in 663, Emperor Gaozong and Empress Wu moved to the newly remodeled palace (which was itself later renamed to Hanyuan Palace). (However, Empress Wang and Consort Xiao continued to appear in her dreams even after this, and therefore, late in Emperor Gaozong's reign, he and Empress Wu were often at the eastern capital Luoyang, not at Chang'an.)

Also in 663, Tufan attacked the Tang vassal Tuyuhun. Tuyuhun's Ledou Khan Murong Nuohebo, unable to withstand the Tufan attack, took his people and fled into Tang territory to seek protection, thus ending Tuyuhun's existence as a state.

Meanwhile, also in 663, Liu Rengui and Liu Renyuan, in conjunction with Silla's King Munmu and the former Baekje crown prince Buyeo Yung, defeated Buyeo Pung and Japanese forces sent to assist him, at the Battle of Baekgang. Buyeo Pung fled to Goguryeo, ending the Baekje resistance movement. Emperor Gaozong recalled Liu Renyuan, leading Liu Rengui in charge of former Baekje territory, but in 664 sent Liu Renyuan back to Baekje and tried to recall Liu Rengui. Liu Rengui petitioned to remain to prepare for another attack on Goguryeo, and Emperor Gaozong agreed to let him remain.

By 664, Empress Wu, who felt that her power was well established, was extending her influence further in the political arena, and when the eunuch Wang Fusheng (王伏勝) reported to Emperor Gaozong that she had engaged the sorcerer Guo Xingzhen (郭行真) – an act that was strictly forbidden—Emperor Gaozong, in anger, summoned the chancellor Shangguan Yi to consult Shangguan. Shangguan suggested that he depose Empress Wu. He agreed, and had Shangguan draft an edict to that effect. However, Empress Wu had received information that that was happening, and she emerged to defend herself. Emperor Gaozong could not carry out the removal, and instead blamed Shangguan. As both Shangguan and Wang had previously served the former crown prince Li Zhong, Empress Wu had Xu Jingzong falsely accuse Shangguan, Wang, and Li Zhong of conspiring against Emperor Gaozong's life. Around the new year 665, Shangguan and Wang were executed, and Li Zhong was forced to commit suicide. From this point on, whenever Emperor Gaozong presided over imperial meetings, Empress Wu would sit behind a pearl screen behind him to hear the reports as well, and political power largely fell into her hands. She and Emperor Gaozong were thereafter referred to as the "Two Saints."

In 665, Emperor Gaozong went to Luoyang and began preparation in earnest to make sacrifices to heaven and earth at Mount Tai – a traditional ceremony for emperors that were rarely carried out in history due to the large expenses associated with them. At Empress Wu's request—as she reasoned that the sacrifice to earth also included sacrifices to past empresses (Emperor Gaozong's mother Empress Zhangsun and grandmother Duchess Dou, posthumously honored as an empress), she believed that it would be more appropriate to have females offer the sacrifices rather than male officials, as had been tradition in the past. Emperor Gaozong decreed that the male ministers would offer sacrifices first, but Empress Wu would next offer sacrifices, followed by Princess Dowager Yan, the mother of Emperor Gaozong's younger brother Li Zhen the Prince of Yue. In winter 665, Emperor Gaozong departed Luoyang and headed for Mount Tai. On the lunar new year (10 February 666[9]), he initiated the sacrifices to heaven, which were not completed until the next day. On 12 February, sacrifices were made to earth. He gave general promotions to the imperial officials, and it was said that starting from this time, promotions of imperial officials, which were strict and slow during the reigns of Emperors Gaozu and Taizong, began to become more relaxed and often excessive. He also declared a general pardon, except for long-term exiles.

In summer 666, Yeon Gaesomun died and was initially succeeded as Dae Mangniji by his oldest son Yeon Namsaeng. As Yeon Namsaeng subsequently carried out a tour of Goguryeo territory, however, rumors began to spread both that Yeon Namsaeng was going to kill his younger brothers Yeon Namgeon and Yeon Namsan, whom he had left in charge at Pyongyang, and that Yeon Namgeon and Yeon Namsan were planning to rebel against Yeon Namsaeng. When Yeon Namsaeng subsequently sent officials close to him back to Pyongyang to try to spy on the situation, Yeon Namgeon arrested them and declared himself Dae Mangniji, attacking his brother. Yeon Namsaeng sent his son (later known as Cheon Heonseong/Quan Xiancheng (泉獻誠), as Yeon Namsaeng changed his family name from Yeon (淵) to observe naming taboo for Emperor Gaozu, whose personal name was the same character) to Tang to seek aid. Emperor Gaozong saw this as the opportunity to destroy Goguryeo, and he initially commissioned Qibi Heli to aid Yeon Namsaeng, and also sent the generals Pang Tongshan (龐同善) and Gao Kan (高侃) to attack Goguryeo.

Meanwhile, Empress Wu's sister, the Lady of Han (who had married the low-level official Helan Yueshi (賀蘭越石)), and her daughter Lady Helan had both been frequently visiting the palace and were said to be "favored" by Emperor Gaozong. When Lady of Han died in 666, Emperor Gaozong created Lady Helan the Lady of Wei, and wanted to let her live in the palace, but hesitated because he thought Empress Wu would be jealous. When Empress Wu heard this, she was indeed jealous, and therefore she poisoned meat offered by her nephews Wu Weiliang (武惟良) and Wu Huaiyun (武懷運), who had been on poor relations with her and whose grandmother were not her mother Lady Yang; she then gave the poisonous meat to Lady Helan, who ate it and died. Empress Wu then implicated Wu Weiliang and Wu Huaiyun in Lady Helan's death and executed them.

Around the new year 667, Emperor Gaozong further commissioned Li Ji to be the overall commander of the attack on Goguryeo, assisted by Hao Chujun. In fall 667, Li Ji crossed the Liao River and captured Sinseong (新城, in modern Fushun, Liaoning). The Tang forces thereafter fought off counterattacks by Yeon Namgeon and joined forces with Yeon Namsaeng, although they were initially unable to cross the Yalu River. In spring 668, Li Ji turned his attention to Goguryeo's northern cities, capturing the important city Buyeo (扶餘, in modern Siping, Jilin). In fall 668, he crossed the Yalu and put Pyongyang under siege. Yeon Namsan and King Bojang surrendered, and while Yeon Namgeon continued to resist in the inner city, his general, the Buddhist monk Shin Seong (信誠) turned against him and surrendered the inner city to Tang forces. Yeon Namgeon tried to commit suicide, but was seized and treated. This was the end of Goguryeo, and Tang annexed Goguryeo into its territory, with Xue Rengui being put initially in charge of former Goguryeo territory as protector general. However, there was much resistance to Tang rule (fanned by Silla, which was displeased that Tang did not give the Goguryeo territory to it), and in 669, following Emperor Gaozong's order, a part of the Goguryeo people were forced to move to the region between the Yangtze River and the Huai River, as well as the regions south of the Qin Mountains and west of Chang'an, only leaving old and weak inhabitants in the original land.

In summer 670, Tufan captured the 18 prefectures that Tang had established over the Xiyu region, and Emperor Gaozong, in response, commissioned Xue Rengui, assisted by Ashina Daozhen (阿史那道真) and Guo Daifeng (郭待封), to attack the Qinghai Lake area, to try to open a second front against Tufan as well as to try to restore the territory previously held by Tuyuhun. However, Guo, who felt himself to be Xue's equal as a general, was displeased at serving as an assistant, and their discord eventually led to a major defeat at the hands of Tufan's prime minister Lun Qinling (論欽陵). For the time being, the Xiyu territory was lost. By 672, Emperor Gaozong gave up the hopes of reestablishing Tuyuhun and moved Murong Nuohebo and his people deep into Tang territory.

By 674, Emperor Gaozong was displeased at King Munmu of Silla encouragement of Goguryeo revival movements who continued to resist Tang rule over the region and he stripped King Munmu of all Tang-bestowed titles, including the title of King of Silla, and arbitrarily conferred them on King Munmu's brother Kim Immun (金仁問) instead, commissioning Liu Rengui, assisted by Li Bi (李弼, Li Ji's brother) and the ethnically Mohe general Li Jinxing (李謹行), to escort Kim Immun back to Silla territory. However, King Munmu formally apologized and offered tribute, Emperor Gaozong ordered a withdrawal and recalled Kim Immun.

The Sassanian prince Peroz III fled to China during this time around 670. The sister of Peroz III was married to the Chinese Emperor, who allowed Sassanian refugees fleeing from the Arab conquest to settle in China.[10]

In 675, Li Jinxing reached Silla territory with Mohe forces that submitted to Tang. However, Tang forces were repelled by Silla army at Maeso fortress.

Meanwhile, by this time, Emperor Gaozong was continuously suffering from a persistent headache, and he considered making Empress Wu regent. Both Hao Chujun and Li Yiyan objected strenuously, and Emperor Gaozong did not make her regent. (The modern Chinese historian Bo Yang suggested that Emperor Gaozong's illnesses may be the result of long-term poisoning by Empress Wu,[11] but did not provide evidence.) However, this did not prevent Empress Wu from engaging a number of mid-level officials, including Yuan Wanqing (元萬頃) and Liu Yizhi, known as the "North Gate Scholars" (北門學士, as the palace sat north of the imperial government) to serve as her advisors and to divert the powers of the chancellors.

Also in 675, Li Hong the Crown Prince died suddenly—with traditional historians largely attributing his death to poisoning by Empress Wu, due to her anger that he tried to curb her power grab and had shown sympathy to his two older sisters born of Consort Xiao—Princesses Yiyang and Gao'an. Emperor Gaozong, in grief, posthumously honored Li Hong as an emperor, and he created Empress Wu's second son, Li Xian the Prince of Yong, crown prince.

Along with the war with Silla, some resistance of Goguryeo people continued to plague Tang rule there. In 676, Xue Rengui crossed the Yellow Sea to against Silla. However, Silla navy expelled Tang forces on the coast of western Silla.

In 677, he bestowed on Goguryeo's former King Bojang, Go Jang, the titles of Prince of Chaoxian and commandant of Liaodong, giving him the forcibly removed Goguryeo people. He also bestowed on Buyeo Yung the title of Prince of Daifang and commandant of Xiongjin—with the intent of letting Go Jang and Buyeo Yung to rebuild Goguryeo and Baekje as loyal vassals to counteract Silla. To accommodate these movements, the Protectorate General was further moved to Xincheng. (Buyeo Yung was not able to make any headway in leading his people back to Baekje territory, while Go Jang tried to break away from Tang rule in alliance with the Mohe; when this was discovered, Emperor Gaozong recalled him to Chang'an and exiled him to Qiong Prefecture (邛州, part of modern Chengdu, Sichuan).)

Meanwhile, Tang had to endure multiple attacks from Tufan, and in 677, he commissioned Liu Rengui, then a chancellor, to take up defense position at Taohe Base (洮河軍, in modern Haidong Prefecture, Qinghai), to prepare a counterattack against Tufan. However, Liu's proposals were repeatedly blocked by another chancellor, Li Jingxuan. In order to retaliate against Li Jingxuan, even though Liu knew that Li Jingxuan was not capable in military matters, he nevertheless recommended that Li Jingxuan take over for himself, and despite Li Jingxuan's attempt to decline, Emperor Gaozong commissioned Li Jingxuan to command the army against Tufan. In fall 678, Li Jingxuan's forces engaged Tufan forces commanded by Lun Qinling, and was soundly defeated, with Li Jingxuan's assistant Liu Shenli (劉審禮) captured by Tufan. Li Jingxuan was only able to escape after being protected by the ethnically Baekje general Heichi Changzhi.

Meanwhile, the Western Tujue chieftain Ashina Duzhi had claimed the title of Shixing Khan and was allied with another chieftain, Li Zhefu (李遮匐), and they pillaged Anxi (i.e., Suyab). Emperor Gaozong was ready to commission an army against Ashina Duzhi and Li Zhefu, but the official Pei Jingxian (裴行儉), who had previously served as secretary general at Xi Prefecture (西州, roughly modern Turfan Prefecture, Xinjiang), opposed—instead proposing that a small detachment be announced to escort the Persian prince Narsieh back to Persia to contend for regal title. Emperor Gaozong agreed and put Pei in charge of the detachment. When Pei arrived at Xi Prefecture, he, under the guise of a hunting party, summoned a number of tribal chiefs loyal to Tang in the area, and once they arrived, launched a surprise attack on Ashina Duzhi, who, caught by surprise, was forced to surrender; Li Zhefu then surrendered as well. (Narsieh, having been used in this manner, was left in the region to fend for himself.)

Meanwhile, though, another trouble was flaring up for Tang. The former Eastern Tujue territory, over which Tang had established 24 prefectures with the tribal chiefs serving as prefects, rebelled, and, under the leadership of Ashide Wenfu (阿史德溫傅) and Ashide Fengzhi (阿史德奉職), supported Ashina Nishoufu as khan. Emperor Gaozong sent Xiao Siye to attack Ashina Nishoufu, but after Xiao achieved a few victories, he became careless, and a counterattack by Ashina Nishoufu crushed him, advancing as far as Ding Prefecture. Ashina Nishoufu also encouraged the Xi and the Khitan to attack Ying Prefecture (營州, roughly modern Zhaoyang, Liaoning), although those attacks were fought off. Around the new year 680, after Pei returned from the Western Tujue front, Emperor Gaozong commissioned him to attack the Eastern Tujue. Pei defeated and captured Ashide Fengzhi, and Ashina Nishoufu's subordinates killed him and surrendered, ending the rebellion.

Meanwhile, Li Xian's relationship with Empress Wu was deteriorating, as Li Xian heard rumors that he was not born of Empress Wu but her sister Lady of Han, and was fearful. Meanwhile, earlier, Ming Chongyan (明崇儼), a sorcerer trusted by Empress Wu and Emperor Gaozong, had repeatedly stated that he believed Li Xian did not have the abilities to be emperor, that his younger brother Li Zhe the Prince of Ying had an appearance like Emperor Taizong, and that another younger brother, Li Dan the Prince of Xiang, had the most honored appearance of all. Knowing that Li Xian was fearful of her, Empress Wu had the North Gate Scholars author teachings on filial piety to give to Li Xian, and also personally rebuked him, further causing him to be alarmed. When Ming was assassinated in 679, Empress Wu suspected Li Xian of carrying out the assassination. She had a report made to Emperor Gaozong that accused Li Xian of assassinating Ming. When the officials Xue Yuanchao, Pei Yan, and Gao Zhizhou were put in charge of investigations, they discovered a number of armors in Li Xian's palace. Empress Wu thus accused Li Xian of treason, and while Emperor Gaozong initially wanted to take no actions against Li Xian, at Empress Wu's insistence he relented. In fall 680, he reduced Li Xian to commoner rank, and created Li Zhe as the new crown prince.

In 682, the Western Tujue chieftain Ashina Chebo (阿史那車薄) rebelled against Tang rule, and Emperor Gaozong initially was to commission Pei again, but before the army could depart, Pei died. However, the commandant at Anxi, Wang Fangyi (王方翼, Empress Wang's cousin) was able to defeat Ashina Chebo and crush the rebellion.

Also in 682, the Eastern Tujue chief Ashina Gudulu rose, in alliance with Ashide Yuanzhen (阿史德元珍), to claim khan title. This, unlike the several earlier rebellions, actually saw the Eastern Tujue khanate being permanently reestablished to Tang's north and persisting for decades, and would plague the last two years of Emperor Gaozong's reign as well as the reigns of his successors.

Late in 683, Emperor Gaozong was seriously ill, and he, who was then at Luoyang, summoned Li Zhe, then in charge of Chang'an, to Luoyang to attend to him. On 27 December, 683, he died. Li Zhe succeeded him (as Emperor Zhongzong), but actual power would be in the hands of Empress Wu, who became empress dowager.

Known by Islamic sources as Yung Wei,[citation needed] which was in fact the name of the first era in his reign (Yonghui era from February 650 to February 656; see era name), Islamic sources credit him with building the first mosque, a mosque that still stands in Guangzhou.[12] According to those records, Islam was introduced to China and Emperor Gaozong by the visit of Sa`d ibn Abi Waqqas, a companion of the Prophet Muhammad, in the year 650. According to these sources, Emperor Gaozong is said to have respected the teachings of Islam greatly, feeling the teachings were compatible with Confucianism, and offered the building of the mosque as a sign of admiration. The emperor himself did not convert as he felt Islam was too restrictive for his own preferences, but according to those sources, did not stop him from allowing Sa`d and his company to spread the teachings throughout the region. These sources, however, were not corroborated by Chinese records.

Li Shangjin (李上金), originally the Prince of Qi (created 650), later the Prince of Bi (created 684), later the Prince of Ze (created 684, committed suicide 690)

Li Sujie (李素節) (b. 645), originally the Prince of Yong (created 650), later the Prince of Xun (created 656), demoted to Prince of Poyang (demoted 676), later the Prince of Ge (created 684), later the Prince of Xu (created 684, executed 690)

Li Hong (李弘) (b. 652), originally the Prince of Dai (created 653), later the Crown Prince (created 656, died 675), posthumously honored Emperor Xiaojing with the temple name Yizong

Li Xián (李賢) (note different tone than his brother) (b. 653), name changed to Li De (李德) 672, changed back to Li Xián 674, originally the Prince of Lu (created 655), later the Prince of Pei (created 661), later the Prince of Yong (created 672), later the Crown Prince (created 675), later demoted to commoner rank (demoted 680, forced to commit suicide 684), posthumously initially honored the Prince of Yong, later honored Crown Prince Zhanghuai

Li Xiǎn (李顯) (note different tone than his brother) (b. 655), name changed to Li Zhe (李哲) 677, changed back to Li Xiǎn 698, changed to Wu Xian (武顯) 700, changed back to Li Xian 705, initially the Prince of Zhou (created 656), later the Prince of Ying (created 677), later the Crown Prince (created 680), later Emperor Zhongzong of Tang (enthroned 684), later demoted to Prince of Luling (demoted 684), later the Crown Prince (created 698), later emperor again (705)

Li Dan (李旦), né Li Xulun (李旭輪) (b. 662), name changed to Li Lun (李輪) 669, changed again to Li Dan 678, changed again to Wu Lun (武輪) 690, changed again to Wu Dan (武旦) 698, changed back to Li Dan 705, originally the Prince of Yin (created 662), later the Prince of Yu (created 666), later the Prince of Ji (created 669), later the Prince of Xiang (created 675), later the Prince of Yu (created 678), later Emperor Ruizong of Tang (enthroned 684), later demoted to Crown Prince (demoted 690), later demoted to Prince of Xiang (demoted 698), later emperor again (710)

^The modern historian Bo Yang, based on the fact that Empress Wu's oldest son Li Hong was born in 652, fixed the date of this incident as 650, but 651 is also a possibility. See Bo Yang Edition of Zizhi Tongjian, vol. 47.